Fluorescent whitening agent is an organic compound that can absorb ultraviolet light and excite blue or blue-violet fluorescence. It adsorbs substances with fluorescent whitening agents, and on the one hand, it can reflect visible light shining on objects. At the same time, it can also transform the absorbed ultraviolet light (wavelength 300nm~40Onm) into blue or blue-violet visible light and emit it. Blue and yellow complement each other, thus eliminating the yellow color in the object matrix, It makes it appear white. On the other hand, because the emissivity of the object to the light is increased, the intensity of the emitted light exceeds the intensity of the original visible light projected on the object to be treated. Therefore, people use their eyes to look at the whiteness of the object to increase, so as to achieve the purpose of whitening and beautifying.

From a chromatic perspective, light that produces non colored (colorless) visual reactions is commonly referred to as white light. White is a very special color with high brightness and extremely low saturation. The wavelength range of visible light is between 400 and 760nm, which is only a very narrow section of the solar spectrum. People often mistakenly believe that white light is the sum of all wavelengths of light within the visible spectrum range. In fact, white light with a given whiteness can be generated from two or more monochromatic mixed lights, and monochromatic mixed lights containing two wavelength components can also form white light, and the colors of these two lights are called complementary colors.

Surround the visible spectrum into a circular ring and divide it into nine regions, called the complementary ring (as shown in Figure 1-2). The numbers on the complementary color ring represent the wavelength of the corresponding colored light, in nanometers (nm). The colors in any two opposing positions of the sector on the complementary color ring are called complementary colors. For example, the complementary color of blue (435 nm~480nm) is yellow (580nm~595nm).